1. Field of the Invention
The present invention relates to an optical wavelength division multiplexing network.
2. Description of the Related Art
For a future multimedia network, a super-long and large capacity optical communications system and a light wave network using the system is demanded with research and development proceeding vigorously.
A conventional system for realizing large-capacity data communications can be a time-division multiplexing (TDM) system, an optical time-division multiplexing (OTDM) system, a wavelength-division multiplexing (WDM) system, etc.
Among these systems for realizing the functions of the above described light wave network, the WDM system can utilize the broadband and large capacity of optical fibers, and can select, branch, and insert an optical transmission signal independent of a modulation system or speed using an optical wavelength multiplexer/demultiplexer (optical filter).
That is, the light wave network requires an add/drop multiplexer (ADM) for adding/branching signals as necessary, and optical routing and cross-connecting functions for selecting a transmission line.
An add/drop multiplexer has been studied and developed for adding/branching an optical signal. The add/drop multiplexer can be a fixed-wavelength type for adding/branching optical signals having fixed wavelengths; and an optional wavelength type for adding/branching optical signals having optional wavelengths.
A device of the fixed wavelength type includes, for example, a circulator and a fiber grating, and reflects one of the transmitted optical signals, which has a specific wavelength, on the fiber grating to branch it through the circulator. When an optical signal is added, the optical signal to be added is temporarily transmitted to the fiber grating through the circulator. A specific wavelength is reflected on the fiber grating, and the optical signal is multiplexed with an optical signal passing through a transmission line.
In such a device of the fixed wavelength type, the wavelengths of added/branched optical signals are determined when the system is produced. Consequently, there is the problem that a large number of requests to the light wave network cannot be completely satisfied.
On the other hand, since the wavelengths of added/branched optical signals in a device of the optional wavelength type can be changed through a remote operation even after the system has been produced, a request to change added/branched wavelengths (channels) can be easily satisfied.
FIG. 1 shows an example of the configuration of an optical ADM device using an optical switch.
A wavelength multiplexed light having the wavelengths xcexl through xcexn is input from the input terminal to a demultiplexer (DMUX), and is branched into optical signals having respective wavelengths. An optical signal having each wavelength is input to a 2xc3x972 optical switch provided for each wavelength. The 2xc3x972 optical switch passes each optical signal through, or drops it.
The optical signal dropped by the 2xc3x972 optical switch is transmitted to a tributary station (branch station). An optical signal passing through the 2xc3x972 optical switch is input to a multiplexer as is, multiplexed into a wavelength multiplexed light, and then output. An optical signal dropped by the 2xc3x972 optical switch is transmitted to a tributary station. The tributary station first multiplexes the dropped optical signal through a wavelength multiplexer/demultiplexer, and then branches the multiplexed optical signal to provide an optical signal to an optical receiver OR provided for each channel. Although not shown in FIG. 1, the optical receiver OR is provided with a wavelength selection filter, selects an optical signal having a predetermined wavelength from among optical signals branched by a wavelength multiplexer/demultiplexer, and receives the selected signal.
Thus, an optical signal having a specified wavelength can be dropped by demultiplexing, by the OADM device, the optical signal wavelength-multiplexed into signals of respective wavelengths and dropping each optical signal. At the terminal of a tributary station, an optical signal of a specified wavelength (channel) can be received by selecting a desired wavelength from among dropped optical signals and receiving an optical signal having the selected wavelength. Especially, when dropped wavelengths are different from each other, the wavelength of an optical signal received by, for example, the first optical receiver can be variable if a wavelength selection filter provided before the optical receiver OR can select variable wavelengths.
An electric signal converted from an optical signal by an optical receiver OR is processed by an electric ADM device (E ADM) for performing an add/drop multiplexing using an electric signal. A signal to be transmitted from a tributary station is output from the E ADM, and is converted into an optical signal by an optical transmitter OS for output. The wavelength of the optical signal output from each of the optical transmitters OS of the tributary station shown in FIG. 1 is one of the wavelengths dropped by the OADM device, and is output to an optical switch. An optical switch switches the optical path of an optical signal transmitted from an optical transmitter OS, and transmits an optical signal having a corresponding wavelength to a 2xc3x972 optical switch which performs a dropping process. Each 2xc3x972 optical switch for performing a dropping process receives an optical signal having the same wavelength as the dropped optical signal from the tributary station, and transmits the signal to a multiplexer MUX. Thus, the optical signal transmitted from the tributary station is multiplexed with the optical signal passing through the OADM device, and is output as a wavelength multiplexed optical signal.
An OADM device of an optional wavelength type can normally be the above described device using an optical switch. However, it does not operate quickly. Furthermore, when an optical network is operated by a system using a smaller number of wavelengths than the maximum number of multiplexed wavelengths, it has output/input ports of a multiplexer and a demultiplexer, which are not required, and therefore has unnecessary equipment. Additionally, when a 2xc3x972 optical switch is equipped from the beginning, it is an unnecessary optical switch consuming the initial investment.
In the above described system, since an optical signal is branched by the multiplexer to optical signals having each wavelength, the multiplexer has the characteristic of a band pass filter for optical signals having each wavelength. If devices having such a characteristic of a band pass filter are connected in series, small differences in pass band are accumulated and cause the problem that the pass band of the entire system becomes very narrow for each wavelength. Therefore, to solve the problem, the pass bands of optical devices should strictly match each other, thereby imposing severe restrictions on the system design and mounting operations.
Furthermore, since the optical signal is AM-modulated, a side band is generated in the component of a wavelength. If such an optical signal is propagated through a system having a very narrow pass band, then the wavelength is undesirably varied, and the receiving unit may not be able to receive an optical signal. In the worst case, the system cannot propagate an optical signal.
The above described problem occurs when the system is designed such that all wavelengths are temporarily demultiplexed by a multiplexer/demultiplexer, etc. Therefore, when a fiber grating is used as in a device of a fixed wavelength type, only an optical signal having a dropped wavelength is removed and the characteristic of the fiber grating for the components of other wavelengths is flat. As a result, there is not the above described problem that the pass band is narrow for the entire system.
Therefore, the OADM device can be designed using a fiber grating. However, since the fiber grating itself is fixed to a selected wavelength, one is required for each wavelength and an optical switch is also required for each fiber grating when an OADM device of an optional wavelength type is designed, thereby generating a slow device.
Furthermore, since the OADM device must cooperate with an electric ADM device to process a signal, the system is costly when an electric ADM device is initially provided for each wavelength. Therefore, the system should be designed such that the sum of the cost of the electric ADM device to be provided and the cost of the OADM device can be as small as possible.
In response to the request to increase the number of multiplexed wavelengths, small switches may have to be combined to construct a large scale switch, since matrix switches required to process the wavelengths for 32 wavelengths are not available. In this case, however, a scale of a switch becomes very large, and it is undesirable when considering a downsizing of the equipment of an OADM system.
To solve the above described problem, an acoustooptic tunable filter (AOTF) can be used. Since the AOTF extracts only the light having a dropped wavelength in the same manner as a fiber grating, the wavelength characteristic for the optical signal is flat, thereby solving the above described problem that the pass band is narrow for the entire system. Furthermore, unlike the fiber grating, a wavelength to be dropped is optionally selected. Consequently, the OADM device of the optional wavelength type can be easily designed. Furthermore, since the AOTF can be used as a wavelength selection filter, the band pass filter of the fixed transmission wavelength type can be replaced with the AOTF as a wavelength selection filter of a tributary station. Thus, it is a device applicable in many fields, inexpensive, and appropriate for use in an OADM device.
The present invention aims at providing an optical wavelength multiplexed network and a device which are reliable using an AOTF, and excel in cost-effectiveness.
The optical transmission apparatus according to the present invention in a WDM communications system branches and adds an optical signal having an optional wavelength, and includes at least two variable wavelength selection filters, that is, a first variable wavelength selection filter for branching and adding a part of optical signals to be branched and added; and a second variable wavelength selection filter for branching and adding the optical signals which are to be branched and added, but have not been selected by the first variable wavelength selection filter. With this configuration, the optical transmission apparatus branches or adds all optical signals to be added and branched using a plurality of variable wavelength selection filters.
The optical terminal station according to the present invention receives an optical signal branched by the optical transmission apparatus for branching and adding an optical signal to be branched and added, and transmits an optical signal to be added to the optical transmission apparatus in a WDM optical communications system. The optical terminal station includes a wavelength multiplexer/demultiplexer for multiplexing a requested number of optical signals having a predetermined wavelength, and transmitting them as optical signals to be added to the optical transmission apparatus.
The optical transmission system according to the present invention includes an optical transmission apparatus for branching an optical signal having a predetermined wavelength in the wavelength multiplexed optical signals transmitted through a transmission line, and for adding an optical signal having a corresponding wavelength; and an optical terminal station for receiving an optical signal branched by the optical transmission apparatus and transmitting an optical signal to be added to the optical transmission apparatus. The optical transmission system further includes an optical amplifier for amplifying the optical signal branched by the optical transmission apparatus as necessary; an optical splitter for splitting the optical signal into a desired number of wavelengths; and an optical filter provided for each output from the optical splitter. With this configuration, the optical terminal station selects and receives a signal having a predetermined wavelength.
The optical transmission system according to another aspect of the present invention operates in an optical network containing an optical transmission apparatus for branching an optical signal from a transmission line or adding an optical signal to the transmission line; and a terminal station for receiving the optical signal branched by the optical transmission apparatus and transmitting an optical signal to be added to the optical transmission apparatus. The optical transmission system performs the following sequential process of: applying a predetermined RF frequency to a single-wave selection AOTF at a receiving terminal of the terminal station; branching a predetermined optical signal by applying the predetermined RF frequency to the branching/adding AOTF in the optical transmission apparatus after confirming that the single-wave selection AOTF enters a stable state; applying the predetermined RF frequency to the single-wave addition AOTF of the terminal station after confirming that the predetermined optical signal has been branched by an optical spectrum monitor; and driving an optical transmitting unit in the terminal station after confirming that the operation of the single-wave AOTF has become stable and the optical signal, which is monitored by the optical spectrum monitor and is to be added, has been controlled to have a predetermined optical wavelength and power.
The optical transmission system according to a further aspect of the present invention includes an optical transmission apparatus for modulating before transmission the optical intensity of a transmission signal of one or more wavelengths, and transmitting the modulated signal in an optically-amplifying multiple relay transmission; and a node, provided in a transmission line for the optical transmission apparatus, having the function of branching and adding a transmission signal light. With this configuration, the optical transmission system further includes a unit for modulating an optical phase or an optical frequency of a transmitted light in a transmitting unit; a transmitter having a positive sign for a chirping parameter of the modulation unit; and a dispersion compensation unit, provided between the transmitter and the transmission line and between the transmission line and a receiver, for compensating for the wavelength dispersion characteristic of a transmission line.
The AOTF control device according to the present invention is provided on the surface of the substrate on which an AOTF is formed. The AOTF selects an optical signal having a predetermined wavelength from a wavelength multiplexed optical signal and adds or branches the selected signal using the function of a surface acoustic wave. The AOTF control device has a resonator near the AOTF and detects a change in the resonant frequency of the resonator so that the surface temperature of the AOTF can be measured, the RF signal can be controlled based on the measurement result, and the operation of the AOTF can be stabilized.
According to the present invention, the operation of the circuit forming part of the system can be quickly performed, and an inexpensive and reliable OADM system can be realized by an AOTF, provided in an add/drop system, capable of selecting an optional optical signal by changing the frequency of an electric signal which is applied for selecting an optional wavelength.